This invention relates to a photoreceptor including, for example, an electrophotographic receptor.
Heretofore, there have so far been well-known electrophotographic receptors such as a photoreceptor comprising Se or Se doped with As, Te, Sb or the like, a photoreceptor comprising ZnO or CdS which is dispersed in a resin binder, and the like. However, these photoreceptors have had problems of environmental contamination, thermal instability and poor mechanical strength.
In recent years, on the other hand, there have been proposals of electrophotographic receptors using amorphous silicon (a-Si) to serve as the parent thereof. a-Si has the so-called dangling-bond when the link of Si-Si is cut-off. As the result of this disadvantage, there are many localized levels in the energy-gap in a-Si. Hopping conduction takes place in a thermoexciting carrier to reduce the dark resistance and a photoexciting carrier is trapped at the localized levels to worsen the photoconductivity. As a countermeasure, the above-mentioned dangling-bond has been filled by offsetting the above-mentioned disadvantages with a hydrogen atom (H) to be linked to Si.
The dark resistivity of such amorphous silicon hydride as mentioned above (hereinafter called a-Si:H) is from 10.sup.8 to 10.sup.9 .OMEGA..cm, that is about one ten-thousandth lower than that of amorphous Se. Therefore, a photoreceptor comprising a single layer of a-Si:H has the problems that it has a relatively greater dark decay rate of surface potential and a relatively lower initial charged voltage. However, on the other hand, it has very excellent characteristics to serve as a light-sensitive layer of photoreceptors, because the resistivity thereof will remarkably be reduced when it is irradiated with visible and infra-red rays of light.
FIG. 14 illustrates an electrophotographic copying machine into which an a-Si type photoreceptor having the parent body of the above-mentioned a-Si is incorporated. According to the copying machine, there are arranged on the upper part of cabinet 1 thereof with a glass-made original platen 3 for placing an original document 2 thereon and a platen-cover 4 for covering the original document 2. Below the original platen 3, there is arranged an optical scanning table comprising first-mirror unit 7 provided with light source 5 and mirror 6 for first reflection so that it can reciprocate straight from side to side in the drawing, and second mirror unit 20 for fixing an original document scanning point and the optical path of the photoreceptor is moved according to the reciprocating speed of the first mirror unit 7, so that reflected light from original document platen 3 may be incident in slit-like form upon a photoreceptor drum 9 which serves as an image carrier, through lens 21 and reflection mirror 8. Around the drum 9, there are arranged with corona-charger 10, developer 11, image transfer device 12, separation device 13 and cleaning device 14, respectively. Copy papers 18 transferred from paper-feed tray 15 through each of paper feed rollers 16 and 17 and are fixed in fixing device 19 after an image registered on the drum 9 is transferred onto the copy paper 18 and then the copy paper 18 is delivered from the copying machine. In the fixing device 19, a fixing operation is made by passing the developed copy paper through between heat-roller 23 comprising a heater 22 and pressure roller 24.
However, there have not so far satisfactorily been studied the chemical stability of the surfaces of photoreceptors having the surface made of a-Si:H, such as the influence of exposing the surfaces to air and moisture for a long time, the influence of the chemical species produced by a corona-discharge on the surfaces, and the like. For example, it has already been proven that the surfaces thereof allowed to stand for not less than one month are affected by moisture and the incoming voltage thereto is also lowered seriously. On the other hand, there are descriptions in `Phil. Mag.` Vol. 35, 1978 and the like about the preparation processes and the existence of amorphous silicon carbide, carbide hydride (hereinafter called a-SiC:H) and it has been well-known that the characteristics thereof are substantially higher in heat-resistance and surface-hardness and relatively higher in dark-resistance that is from 10.sup.12 to 10.sup.13 .OMEGA..cm than that of a-Si:H, and also the optical energy gap thereof will be varied in accordance with the variations of the carbon content, to the extent of the range from 1.6 to 2.8 eV, while there is a disadvantage that the sensitivity thereof in long wavelengths will become poor because the band-gap thereof will be widened due to the carbon content.
Such an electrophotographic receptor combining a-SiC:H with a-Si:H is proposed in, for example, Japanese Patent Publication Open to Public Inspection (hereinafter called Japanese Patent O.P.I. Publication) No. 127083/1980. According to this Japanese Patent Publication, as improvement of the charged voltage thereof is tried in such a manner that an a-Si:H layer serves as a charge-generating (photoconductive) layer and an a-SiC:H layer is provided underneath the charge-generating layer so as to be photosensitive in a relatively wider wavelength region than the a-Si:H and the charge voltage thereof may be improved by the formation between the a-Si:H layer and the a-SiC:H of the lower layer of a hetero-junction. In such an electrophotographic receptor as described above, however, the dark-decay of the a-Si:H layer cannot be prevented satisfactorily and the charged voltage thereof is still unsatisfactory for putting into practice, and in addition, the chemical stability, mechanical strength, heat-resistance and the like thereof will become poor because an a-Si:H layer is present on the surface thereof.
In the meantime Japanese Patent O.P.I. Publication No. 17952/1982 discloses that a primary a-SiC:H layer is formed on a charge-generating layer comprising a-Si:H so as to serve as a surface modifying layer and, onto the back side thereof, i.e., on the support-electrode side, a secondary a-SiC:H layer is formed.
Further, a photoreceptor relating to the above-mentioned technology is known as described in Japanese Patent O.P.I. Publication No. 23543/1982, in which an inclined layer of a-Si.sub.1-x C.sub.x :H is interposed between the above-mentioned charge-generating layer and the above-mentioned primary and secondary a-SiC:H layers, and in this inclined layer, x is made to be zero (x=0) on the a-Si:H side and x is made to be 0.5 (x=0.5) on the a-SiC:H layer side.
When the inventors studied the above-mentioned well-known photoreceptors, they found that the effects of providing a surface modifying layer could not display so much particularly in a continuous repetition in use. To be more concrete, it was found that when putting such a photoreceptor into a continuous operation of from 200,000 to 300,000 copies the surface of the a-SiC layer thereof is mechanically damaged in the course of operation of the order of from 70,000 to 80,000 copies and by this damage an image defect such as white streaks and white spots are caused, so that the printing durability cannot be satisfactory. In addition to the above, the light-fastness thereof will be lowered with repeating the operation and an blur is also caused, and further the electrical and optical characteristics thereof are not stabilized at any time, and still further it cannot be neglected that such photoreceptors are affected by the conditions of the use such as temperature and humidity. Besides the above, it is required to further improve the adhesive property of a surface modifying layer to a charge-generating layer.